This invention relates generally to magnetic resonance imaging (MR), and more particularly, the invention relates to correction of local field inhomogeneity in MR apparatus.
FIG. 1A is a perspective view partially in section illustrating coil apparatus in an MR imaging system, and FIGS. 1B-1D illustrate field gradients which can be produced in the apparatus of FIG. 1A In operation, a uniform static field B0 is generated by the magnet comprising the coil pair 10. A gradient Gx is generated by a complex gradient coil set which can be wound onto cylinder 12. An RF field B1 is generated by a saddle coil 14. A patient undergoing imaging would be positioned along the Z axis within saddle coil 14. In FIG. 1B an x-gradient field is shown which is parallel to the static field B0 and varies linearly with distance along the x axis that does not vary with distance along the y and z axes. FIGS. 1C and 1D are similar representations of the y gradient and z gradient fields, respectively.
Conventionally, MR apparatus includes shim coils to correct main field, B0, inhomogeneity due to manufacturing tolerances and the like which can disturb the field. It is also known to provide external coils around the MR coils to counteract fields external to the MR apparatus. See U.S. Pat. No. 4,595,899, for example. These prior art shim coils are generally placed around the entire imaging volume.
The role of imaging in medicine has recently expanded, with increased emphasis on imaging during interventional procedures, combined modality imaging (i.e., x-ray and MR simultaneously), and increased patient monitoring during imaging, as illustrated in FIG. 2. Here, two superconducting magnets 20, 22 are coaxially aligned but spaced apart to accommodate an x-ray tube 24 and detector 26 for imaging an object (patient) 28. A problem has heretofore been recognized stemming from magnetic field of the MR apparatus disturbing the x-ray tube at operation. U.S. Pat. No. 4,595,899 addresses this problem by positioning the x-ray tube at a distance from the MR device at an area where the static magnetic stray field is weak enough to be shielded without distorting the static field in the MR device.
A problem has been recognized by applicants herein due to the presence of the x-ray detector in close proximity to the static magnetic field and resulting in an inhomogeneity in the static magnetic field near the detector which can adversely affect MR imaging. The problem can similarly occur with other components placed within or near to the imaging volume. The present invention is directed to overcoming this problem.
In accordance with the invention, an object in close proximity to MR apparatus and causing perturbations in the static magnetic field is provided with magnetic coils which can offset the perturbations in the magnetic field outside of the object. Thus, the effect of any magnetic material in the object, or any magnetic field generated by the object which can distort the main magnetic field is offset with compensating coils. In contrast to prior art methods that place shim coils around the entire imaging volume, the coils of the present invention are purposely positioned near the object causing the magnetic field inhomogeneity.
In a specific embodiment, an x-ray detector placed in the bore of an MR system is provided with a plurality of magnetic coils surrounding the detector. Appropriate current is applied through the coils to correct main field inhomogeneity caused by the detector.
The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawings.